It is often difficult to weld metals that are formed of two different materials. For example, coated steel may be one metal that is difficult to weld—especially when the coating has a lower melting point than the base metal. Different welds of this type, including a lap joint, have been attempted. However, this has been a challenge.
One problem is that during the welding process, the coating may evaporate violently at the weld interface because of the coating's lower boiling point than the base metal. This may cause undesirable effects within the weld joints. This may also lead to defects, including high levels of porosity, as well as expulsion of the base metal from the weld pool.
Different approaches have been attempted to deal with this problem. One of the approaches uses a laser welding technique, with plural sheets, each in a vertical position. The weld is carried out from bottom to top in order to allow gravity to lengthen an area through which the vapors can escape. This approach has been difficult to use in production.
Another technique attempts to remove the coating prior to the weld. However, this requires additional processing, and hence can be relatively expensive.
Gap welding requires a constant joint gap during the weld process and hence may be difficult to maintain. The joint design may also be changed in order to attempt to allow an escape path for the gases from either one or both sides of the joint.